Pretty boys like Zac Ephron and Chace Crawford are modern-day sex symbols, which is quite a switch from the 1950s and 1960s. Back then, the rugged, very masculine look was in vogue, and Sean Connery, Marlon Brandon and Steve McQueen were considered the heart stoppers of the day.

Why the new preference for younger, boyishly smooth types? One study claims that the hormones in the pill actually suppress a woman's interest in very masculine men causing women to be more attracted to boyish types. Think Robert Pattinson versus John Wayne.

The provocative research, which appeared in Trends in Ecology and Evolution, found that when hormonal cycles are altered--which is what takes place when a woman takes the pill--then the choice of mates among both men and women could be altered. Furthermore, this may even be a problem in the future, in terms of successful reproduction, the study found..

"The use of the pill by women, by changing her mate preferences, might induce women to mate with otherwise less-preferred partners, which might have important consequences for mate choice and reproductive outcomes," said Alexandra Alvergne, lead author of the study. "One prediction is that offspring of pill users are more homozygous than expected, possibly related to impaired immune function and decreased perceived health and attractiveness," the report states.

But William Hurd, a reproductive endocrinologist at University Hospitals Case Medical Center in Cleveland, told HealthDay News that the news on the pill just may have been overinterpreted.

"The study was about female preferences in their relation to hormones in the cycle, but that's not the same as your mate selection for a long-term relationship," Hurd said. "If you don't take into account society, maybe we're all animals, but in social situations, I don't think there are many women who change who they would mate with at different times of the month. It might change desires or perceptions, but gee whiz, that's a long stretch to changing who you would date or even who you would go to dinner with."

When ovulating, women tend to want "manly men," i.e., those who are dominant, competitive and have more masculine facial features, according to the study's background information. And genetically speaking, they seem to prefer a man who's not like them. Men, when they have a choice, tend to gravitate toward women who are ovulating.

However, women on oral contraceptives are in a hormonal state that mimics pregnancy. They don't ovulate, so they don't ever have a time of the month when they prefer the more masculine, rugged men. Instead, their preferences focus more on, well, slightly feminine men.

Some 100 million women around the world are taking the pill, according to the study authors, who hope the paper will result in more research.

Meanwhile, Hurd thinks other trends are responsible for how people mate and reproduce.

"Probably the biggest change in my lifetime is how people meet each other: online and using programs that match them up for compatibility," he says. "That's probably going to have a massive effect on how people end up dating and ultimately reproducing. Just because you like someone with a square jaw in the middle of your cycle probably doesn't affect who you end up with."